Recently, in access networks for providing multimedia service to respective homes, point to multi-point access optical communication systems called passive optical network (PON) systems realized by a public network using optical fiber are being broadly used.
A PON system is constituted by one optical line terminal (OLT) which is a station-side apparatus, and multiple optical network units (ONUs), which are termination devices for subscribers, connected via optical star couplers. This is advantageous because, since most of the optical fiber connecting medium and the OLT may be shared by multiple ONUs, a reduction in operating costs can be expected, and since there is no need to supply power to the optical star coupler, which is a passive component, outdoor installation is easy, and reliability is high. Given these advantages, PON systems are actively being introduced as optical communication systems that realize broadband networks.
In such a PON system, since each ONU is positioned at a different distance from the OLT, at the OLT the optical signal levels of the optical signals transmitted from each of the ONUs differ for each received packet that the OLT receives from each ONU. Consequently, wide dynamic range that consistently reproduces packets at different optical signal levels is demanded of the optical receiver in the OLT. A preamplifier installed in the optical receiver is typically equipped with an automatic gain control (AGC) circuit in order to realize wide dynamic range.
For example, in the optical receiving device disclosed in Patent Literature 1, the conversion gain of a preamplifier is actively switched per packet. However, such a method requires components such as a selecting circuit that selects a feedback resistor for setting the conversion gain to an optimal value according to the optical signal level, and the circuit size is increased. Furthermore, since switching is conducted per packet, converging onto an optimal conversion gain takes time.
In contrast, the optical receiver disclosed in Patent Literature 2, for example, passively varies the conversion gain of a preamplifier according to the optical signal level. Namely, the conversion gain is passively varied according to the intensity of a photocurrent converted by a photodetector. Specifically, the preamplifier of the optical receiver is equipped with a current-to-voltage conversion circuit that converts a photocurrent corresponding to the optical signal level into a voltage signal. The current-to-voltage conversion circuit includes an amplifier and a feedback resistor. A diode or the like is connected in parallel to the feedback resistor of the current-to-voltage conversion circuit.
In such an optical receiver, when the optical signal level rises and the photocurrent generated by the photodetector increases, the voltage drop across the feedback resistor increases, and the input/output voltage difference of the current-to-voltage conversion circuit increases. Subsequently, when the input/output voltage difference of the current-to-voltage conversion circuit exceeds the threshold voltage of the diode, current flows in the diode, and the overall resistance of the feedback resistor and the diode connected in parallel decreases. Consequently, since the conversion gain of the current-to-voltage conversion circuit decreases, passively varying the conversion gain according to the photocurrent becomes possible.
In other words, when a photocurrent is large, by passing most of the photocurrent to the diode having an approximately constant voltage drop, the voltage drop produced at the feedback resistor is moderated, and a received signal that is a voltage signal can be output even for input light with a high optical signal level, thereby realizing wide dynamic range. Also, since the preamplifier of Patent Literature 2 can conduct AGC operation per 1 bit of an optical digital signal, high-speed operation becomes possible.